The probability that an electron is transferred from the left electrode to the right electrode is a function of the energy (as we see in the transmission spectrum) but also of the electron's incident momentum
k = (kx,ky). Electrons with kx=ky=0 are traveling parallel to the z axis, but for other values of kx and ky, the electron is traveling at an angle, determined by the projection of the vector
k onto the z axis.
I think slide 13 of Part 2 of the presentation "ATK Tutorial (What Everyone Should Know About ATK)" at
http://quantumwise.com/tutorials will be useful for you.
